This work centers on the high molecular weight, high mobility group (HMG), nonhistone chromatin proteins. We have found that in cultures of rapidly proliferating cells HMG-17 requires much higher NaCL concentrations for dissociation from chromatin than is required for its dissociation from chromatin of quiescent cells. This suggests a possible involvement of this protein in DNA replication. We have shown that HMG-1 and HMG-2 occur in cultured hepatoma cells in much higher amounts than in adult rat liver and that both proteins possess preferential affinity for single-stranded DNA. We discovered a three-domain organization of HMG-1 and HMG-2. Two of the domains are sequence homologous, globular, DNA-binding domains. The third domain is probably a random coil that binds to histones. The proteins are thus peculiarly adapted to interact through separate domains with both DNA and histones. We have searched for nonhistone chromosomal proteins that bind to single-stranded DNA in the presence of dextran sulfate. This polyanionic reagent has been suggested by other workers to dissociate proteins that bind nonspecifically to single-stranded DNA. Dextran sulfate-resistant proteins, several of which we have now identified in cultured L cells and none of which are HMG proteins, are presumed to have a more specific interaction with single-stranded DNA and are more likely to interact with single-stranded DNA in vivo. Ion-exchange HPLC appears to offer a suitable method for fractionating the dextran sulfate-resistant fraction to obtain individual, purified proteins for intensive study. (K)